1. Technical Field
This invention generally relates to a surface-emitting semiconductor laser element used for a light source for optical data processing or high-speed optical communication, and a manufacturing method thereof.
2. Related Art
Recently, in technical fields such as optical communication or optical storage, there has been a growing interest in Vertical-Cavity Surface-Emitting Laser diode (hereinafter referred to VCSEL).
VCSELs have advantages in that they require lower threshold current and have smaller power consumption than edge-emitting semiconductor lasers, which have been used for laser light sources. Other excellent characteristics which edge-emitting semiconductor lasers do not have are: a round light spot can be easily obtained; evaluation can be performed while VCSELs are on a wafer; and light sources can be arranged in two-dimensional arrays. On the other hand, because the volume of the active region of a VCSEL is small, which leads to low threshold current; it is difficult to obtain high optical output that excess 10 mW with a VCSEL. Another disadvantage is that VCSEL's resistance, which is typically several tens to several hundreds ohms, is significantly higher than edge emitting semiconductor lasers (several ohms).
Optical communication using optical fibers has been used for data transmission mainly for data transmission at middle to long distance (several kilometers to several dozen kilometers) until now. Such transmission has been using single-mode optical fibers made of quartz, and laser that has a lasing peak in the long wavelength region of 1.31 micrometers or 1.55 micrometers. These are light sources that have advantages of their less dispersion in the fibers, or extremely small transmission loss. However, they also have many disadvantages in that thermal control of device maybe required, or the optical-axis alignment between optical fibers and laser may require some effort, for example. In addition, main users are communication carriers and thus products of these light sources for general consumers are manufactured in small quantities, which makes the system itself that uses these light sources expensive.
In these days, thanks to the proliferation of Asymmetric Digital Subscriber Line (ADSL) and Cable Television (CATV), ten to hundred times higher-speed and higher-capacity data transmission than ever before has been achieved, and Internet users have been increasing. With the increase, needs for a still higher-speed and higher-capacity data transmission have been arising even in ordinary households, and optical fibers have been provided to many households.
However, it is uneconomic to use the combination of single-mode optical fibers, which are mainly used for middle to long distance transmission, and Distributed Feedback (DFB) laser for transmitting data at such short distance as between a household and a utility pole, which is typically several to several dozen meters at the longest. For the transmission at such short distance (less than several hundreds meters), it is more cost effective to use a less expensive optical fiber, such as a multi-mode silica fiber or Plastic Optical Fiber (POF). Accordingly, the light source to be used for these multi-mode optical fibers should be inexpensive itself. In addition, it is desirable that it does not require a specific optical system or driving system. Thus, a VCSEL, which satisfies these requirements, is one of promising candidates.
In the technology of Local Area Network (LAN), an indoor network, data transmission rate has been increased from ten megabits per second to hundred megabits per second. Recently some LANs provide one gigabits per second, which is expected to be increased to ten gigabits per second in near future. Electrical wirings that use twisted pair cables, which are made of copper, can accommodate up to one gigabits per second. However, it is expected that, for a region beyond one gigabits per second, electrical wirings would face limits in terms of noise resistance and be replaced by optical wirings.
There have been increasing cases to adopt VCSELs for light sources of optical wirings that are used for Ethernet (registered Trademark), which delivers ten gigabits per second, and such VCSELs have been developed. For the modulation bandwidth of as much as several gigahertz, there is no problem at present. However, to further increase the modulation bandwidth, some measure should be taken.
A 3 dB down cut-off frequency (f3 dB), an indicator that shows modulation bandwidth of a semiconductor laser element, can be expressed by the following formula (1) when the inductive reactance is as small as negligible;
                              f                      3            ⁢            dB                          =                  1                      2            ⁢                                                  ⁢            π            ⁢                                                  ⁢            CR                                              (        1        )            where C is the capacitance of the element, and R is the resistance of the element. As can be seen from the formula, modulation bandwidth depends on the CR time constant of the element, and reduction thereof can lead to expansion of bandwidth.
The resistance of the element can be reduced if the diameter of the light emitting region is increased. However, the increase of the diameter of light emitting region inevitably increases the volume of the active region, which impairs response. Therefore, it is found that an easiest way to improve response is to reduce the capacitance of the element.
For the purpose of reducing of the capacitance of the element of a VCSEL, various configurations have been proposed. Typical examples include a polyimide buried structure, or a coplanar electrode structure.
The capacitance C caused between parallel plate conductors can be obtained by the following formula (2);
                    C        =                                            ɛ              0                        ⁢                          ɛ              s                        ⁢            S                    d                                    (        2        )            where ε0 is vacuum permittivity (8.854×10−12 F/m), εs is relative permittivity inherent in the material, S is the area of the conductor, and d is the distance between the conductors.
FIG. 16 is a plan view illustrating a configuration of a VCSEL of a related art having a coplanar electrode structure. The VCSEL includes a light emitting portion 1 having a cylindrical mesa (or post) structure that emits laser light on a substrate, and an annular p-side electrode 2 disposed on a top portion of the mesa. The p-side electrode 2 is connected to a p-side contact layer on a top portion of the mesa through an annular contact region 3 (shown by a hatch pattern). To form a coplanar electrode structure, n-side electrodes 4a and 4b on the ground side are electrically connected to an n-type semiconductor layer 5 (shown by a hatch pattern) below an active layer through a contact hole that extends so as to surround most portion of the light emitting portion 1. To increase contact area between the n-side electrodes 4a and 4b and the n-type semiconductor layer 5, especially in FIG. 16, the contact portion or contact hole of the n-side electrodes 4a and 4b and the n-type semiconductor layer 5 extends over the line L of θ=π (radian) that passes across center of the light emitting portion 1, having a larger angle than the line L. However, it is found that, with such configuration of the contact portion, carriers diffused from the p-side electrode 2 (arrows indicate direction of current flow) lose their way to go in a region 6 below the line L, which causes so-called current crowding. This increases recombination, which does not contribute to light emission, and makes threshold current higher than expected.
In addition, although two electrodes 4a and 4b on the ground side are formed to form the coplanar electrode structure, it is expected that inadequate routing of these electrodes may cause interference between magnetic field lines, resulting in transmission loss.
As such, regarding electrode structures of VCSELs of related arts for the aim of improving high frequency characteristics, there is still room for improvement.